A large amount of an etching agent which contains hydrogen fluoride or hydrogen fluoride and ammonium fluoride as main ingredients has been used in a semiconductor manufacturing process, a related process thereof, or a surface treatment process of various materials including metal materials, single crystal materials, optical materials, and the like. An etching agent which contains hydrogen fluoride as a main ingredient or an etching agent which contains hydrogen fluoride and ammonium fluoride (buffered hydrofluoric acid) as main ingredients contains high levels of fluorine as HF. Therefore, when these etching agents flow into a waste water system, waste water thereof become to contain fluorine at a high concentration. As materials treated with etching agents are washed with a large amount of cleaning water during etching process and after the process, a large amount of waste water containing low levels of fluorine is discharged from the washing process.
According to a conventional method, waste water containing high levels of fluorine and waste water containing low levels of fluorine are mixed and treated. Patent Document 1 discloses a method for treating fluorine-containing waste water wherein the waste water is fed to and passed through a reaction tower packed with granular calcium carbonate.
The method described in Patent Document 1, treatment is performed by a merry-go-round method, in which a plurality of towers packed with calcium carbonate are arranged in series. First, raw water is sequentially passed from a first tower (tower in a first stage) to the following tower(s) to remove and recover fluorine. When the fluorine concentration of the raw water flowing into the first tower and the fluorine concentration of the treated water flowed out from the first tower become almost the same, feeding raw water to the first tower is stopped, the calcium fluoride is recovered from the first tower, and the first tower is packed with fresh calcium carbonate. Then, raw water is fed to and passed through the second tower, and flows sequentially to the following tower(s) for the first tower.
Patent Document 1 describes that granular calcium carbonate packed in each calcium carbonate packed tower has a diameter of about 0.1 to 0.5 mm, and specifically granular calcium carbonate having a diameter of 0.25 mm is used in Examples thereof.
Patent Document 2 discloses a method for removing fluorine from fluorine-containing raw water including a process of feeding the raw water to a reaction tower packed with granular calcium carbonate to remove the fluorine, wherein acid or alkali is added to the raw water based on an α-value calculated from measured values of fluorine concentration and acid concentration in the raw water.